Flash Notifications are used to give Flash Alerts when you get a Call / SMS.

This feature is already there is most phones by going to:

Settings > Accessibility > Hearing > Flash Alerts

Toggle there to turn it ON/OFF.

This is really beneficial in alerting without making any sound.

Don't get dishearted if your phone does not have this feature. There are a number of apps available on Play Store.

I'll personally recommend you to use the app named Flash Notification available in play store. It is really easy to use and you can enable it for particular apps and for different sound profiles(Ring/Vibrate/DND).

You can also watch below video to help you activate the default feature: